1. Technical Field
The invention relates to a capacitive touch panel for comparing the stray capacitances of a plurality of sensing electrodes arranged on an insulating panel, and detecting an input operation made near a sensing electrode based on the sensing electrode that increases in stray capacitance in response to an input unit approaching the sensing electrode.
2. Related Art
A capacitive touch panel is known as a pointing device for designating an item such as an icon displayed on a display of an electronic device. The capacitive touch panel detects an input position in a noncontact manner using the change of an electrostatic capacitance caused in response to an input unit such as a finger approaching an entry screen. The capacitive touch panel can detect an input operation even if the capacitive touch panel is placed on the rear side of the display.
A conventional capacitive touch panel includes a large number of X electrodes and a large number of Y electrodes arranged in a matrix on the front and rear sides of an insulating substrate such that the X electrodes and the Y electrodes cross each other. An electrostatic capacitance changes between each of the X electrodes and a Y electrode crossing each other near a position where an input unit such as a finger approached, thereby detecting the position of an operation on the insulating substrate made by the input unit (as disclosed in paragraphs 0017 to 0031 of the specification and FIG. 1 of Japanese Patent Application Publication No. 2005-337773).
In a capacitive touch panel 100 of this type, as shown in FIG. 4, a predetermined pulse voltage is applied sequentially to a large number of Y electrodes 101 to scan the Y electrodes 101. While the pulse voltage is applied to each of the Y electrodes 101, the voltage of an X electrode 102 that crosses the Y electrode 101 having received the pulse voltage is detected. In response to approach of an input unit such as a finger to an insulating substrate, an electrostatic capacitance changes between the X electrode 102 and the Y electrode 101 that cross each other at a position where input unit is approaching. Accordingly, control means 103 detects the position of the operation on the insulating substrate made by the input unit based on the position of the X electrode 102 having changed in voltage as a result of the change of an electrostatic capacitance, and the position of the Y electrode 101 having received the applied pulse voltage at that time.
In the capacitive touch panel 100 disclosed in Japanese Patent Application Publication No. 2005-337773, the large number of X electrodes 102 and Y electrodes 101 must be disposed on the insulating substrate in order to detect an input unit such as a finger based on the change of an electrostatic capacitance. If an entry screen is increased in area, the number of X electrodes and the number of Y electrodes subjected to detection of the change of an electrostatic capacitance are increased in response to the increase of the input area. This results in a longer scanning cycle during which the crossing points of respective electrodes are scanned, making it impossible to detect an input position in a short time.
In addition to the necessity to provide means for applying a pulse voltage, scanning of the large number of X electrodes 102 and Y electrodes 101 arranged in a matrix requires the use of multiplexers in number corresponding to the numbers of the electrodes in response to the increasing size of an entry screen. There is the problem in that the circuit configuration is complicated and increased in size.
Accordingly, a capacitance determining device is suggested (as disclosed in paragraphs 0014 to 0020 of the specification and FIG. 2 of Japanese Patent Application Publication No. 2009-70004) as means for detecting the change of the stray capacitance of a sensing electrode with a simpler circuit configuration. This capacitance determining device detects an unknown electrostatic capacitance at a position of an input operation based on a time constant between the electrostatic capacitance and a known resistance value. In this capacitance determining device, a sensing resistor R is connected in series or in parallel with a capacitor C of an unknown electrostatic capacitance (stray capacitance) to form an RC time constant circuit. A predetermined voltage Vdd is applied to one end of the sensing resistor R, or one end of the sensing resistor R is grounded. The electric potential of the capacitor C that increases or decreases depending on a time constant rc determined by the electrostatic capacitance c of the capacitor C and the resistance value r of the sensing electrode R is compared with a predetermined reference potential. Then, charging times or discharging times until the reference potential is achieved are compared to determine the magnitude of an electrostatic capacitance.
The stray capacitance of a sensing electrode disposed on an insulating panel (electrostatic capacitance between the sensing electrode and the ground) increases to make charging and discharging times longer in response to approach of an input unit such as a finger. Accordingly, based on the aforementioned principles of detection, charging and discharging times until the electric potential of the sensing electrode becomes the same as a predetermined reference potential is measured, and the measured times are compared with charging and discharging times obtained when no input operation is made, so that an input operation approaching the sensing electrode can be detected.
The aforementioned capacitance determining device disclosed in Japanese Patent Application Publication No. 2009-70004 detects an unknown stray capacitance based on a time constant between the stray capacitance and a known resistance value. In this device, charging and discharging times until a reference potential is achieved differ between sensing electrodes. Accordingly, the stray capacitances of all the sensing electrodes cannot be compared simultaneously by a common time measuring circuit composed of a counter for measuring charging and discharging times, a counter-memory, and the like. Thus, a time measuring circuit should be connected to each sensing electrode, or in order to use the common time measuring circuit, charging and discharging times should be made different between the sensing electrodes. This places an obstacle to widespread use of the capacitive touch panel using a time constant of a stray capacitance.
In view of these facts, the applicant of the present application invented the following capacitive touch panel, and filed a patent application assigned as Japanese Patent Application No. 2009-191948. In this capacitive touch panel, binary signals the binary data of which are inverted when the electric potentials of a plurality of sensing electrodes become the same as a reference potential with respect to a common base point in time are given simultaneously as parallel data from the plurality of sensing electrodes to a parallel input register of a bit number corresponding to the number of the plurality of sensing electrodes. The stray capacitances of the plurality of sensing electrodes are compared at the same time based on time until each bit data of the parallel data is inverted.
However, even in the aforementioned capacitive touch panel, the number of sensing electrodes to be subjected to simultaneous detection of the magnitude of a stray capacitance is limited to the number of input bits of the parallel input register. Accordingly, if the capacitive touch panel has more sensing electrodes in number larger than the number of input bits of the parallel input register, in order to detect the stray capacitances of the sensing electrodes, the sensing electrodes should be divided into specific sensing electrode groups such that charging or discharging time does not overlap therebetween.
The capacitive touch panel using a time constant of a stray capacitance makes charge control by applying a predetermined charge voltage Vdd through a resistor to a sensing electrode at a ground potential. The electric potential Vc of the sensing electrode during this charge control is expressed as follows:Vc=Vdd(1−ε−t/cr)  (1)where r is the resistance value of the resistor connected to the sensing electrode, c is a stray capacitance between the sensing electrode and the ground, t is a time elapsed from a base point in time when the charge voltage Vdd is applied, and ε is a natural logarithm. The electric potential Vc of the sensing electrode becomes substantially the same as the charge voltage Vdd after elapse of a transitional period expressed as t=5cr (for the convenience of description, this is called as a state where the electric potential Vc becomes the same as the charge voltage Vdd in the present specification).
Conversely, discharge is controlled by making a sensing electrode at the charge voltage Vdd be at a ground potential through a resistor. The electric potential Vc of the sensing electrode during this discharge control is expressed as follows:Vc=Vdd×ε−t′/cr  (2)where t′ is a time elapsed from a base point in time when the charge voltage Vdd is achieved. The electric potential Vc of the sensing electrode becomes substantially the same as the ground voltage after elapse of a transitional period expressed as t′=5cr (for the convenience of description, this is called as a state where the electric potential Vc becomes the same as the ground potential in the present specification).
Suppose a case where a reference potential to be compared with the electric potential Vc of a sensing electrode is near the charge voltage Vdd, and the resolution of a comparison circuit for comparing the reference potential with the electric potential Vc of the sensing electrode is low. In this case, in the former charge control, the elapsed time t is near the transitional period and the electric potential Vc of the sensing electrode increases slightly with respect to the elapsed time t, making it difficult to compare the electric potential Vc with the reference potential. Meanwhile, in the latter discharge control, the electric potential Vc of the sensing electrode drops largely with respect to the elapsed time t, so that the electric potential Vc can be compared more accurately with the reference potential. Conversely, suppose a case where the reference potential is near the ground potential, and the resolution of the comparison circuit for comparing the reference potential with the electric potential Vc of the sensing electrode is low. In this case, in the latter discharge control, the elapsed time t is near the transitional period and the electric potential Vc of the sensing electrode drops slightly with respect to the elapsed time t, making it difficult to compare the electric potential Vc with the reference potential. Meanwhile, in the former charge control, the electric potential Vc of the sensing electrode increases largely with respect to the elapsed time t immediately after start of the charge control, so that the electric potential Vc can be compared more accurately with the reference potential.
The conventional capacitance determining device disclosed in Japanese Patent Application Publication No. 2009-70004 uses only one of the controls for detection. Accordingly, the charge voltage Vdd and the reference potential should be adjusted optimally according to each of the control methods. If they are fixed potentials that cannot be adjusted, there is the problem in that detection accuracy of a stray capacitance may be reduced.
The stray capacitance of a sensing electrode changes under the influence of a peripheral circuit element such as a liquid crystal display element or a peripheral unit. Accordingly, while making comparison between the charge control and the discharge control and selecting the control with a higher degree of detection accuracy have been desired in a stage of product evaluation after assembly, change of the control method has not been allowed.
The present invention has been made in light of the aforementioned conventional problems. It is an object of the invention to provide a capacitive touch panel that compares input operations made to a larger number of sensing electrodes in a short period of time by making two types of sensing electrode groups share a time measuring circuit for simultaneously measuring charging and discharging times that depend on the magnitude of a stray capacitance.
It is also an objet of the invention to provide a capacitive touch panel capable of selecting a method of controlling a voltage with a higher degree of detection accuracy according to a charge voltage Vdd and a reference potential to be compared with the electric potential Vc of a sensing electrode.
It is also an object of the invention to provide a capacitive touch panel capable of selecting a method of controlling a voltage with a higher degree of detection accuracy according to an operating environment.